[Bug 275002] if_wg: Missing failure check for m_copyback()

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275002

            Bug ID: 275002
           Summary: if_wg: Missing failure check for m_copyback()
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: aly@aaronly.me

Well, the m_copyback() may need to allocate new mbufs in order to hold the
data, so the allocation may fail because it uses the 'M_NOWAIT' flag and
doesn't provided any parameter to use the 'M_WAITOK' flag for allocation.  (For
the reference, OpenBSD extended m_copyback() to have the extra 'how' parameter
for this purpose.)

As a consequence, the caller should check the resulting packet length for
m_copyback() failure.  Below is the patch to 'if_wg.c' function
'wg_send_buf()', please review.

--- if_wg.c.orig        2023-10-12 09:06:16.983637264 +0800
+++ if_wg.c     2023-10-21 15:29:47.928807521 +0800
@@ -911,6 +911,11 @@ retry:
                goto out;
        }
        m_copyback(m, 0, len, buf);
+       if (m->m_pkthdr.len != len) {
+               m_freem(m);
+               ret = ENOMEM;
+               goto out;
+       }

        if (ret == 0) {
                ret = wg_send(sc, e, m);

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